1. Field of the Invention
The present invention relates to an optical scanning device in which a scanning optical unit that emits a light beam and focuses it on a scanned surface is provided with a temperature-compensation unit that eliminates a scanned-surface focal-point deviation of the light beam focused by the scanning optical unit, due to a temperature change of the scanning optical unit and its neighboring locations. Further, the present invention relates to an image forming apparatus in which the optical scanning device is provided with the temperature-compensation unit.
2. Description of the Related Art
In an image forming apparatus, such as laser printer, digital copier or laser facsimile, a laser scanning device emits a laser beam in accordance with an image signal, and focuses the laser beam onto a scanned surface of a photosensitive medium. In the laser scanning device, a collimator lens converts the laser beam, emitted by the laser light source, into a collimated laser beam, and a rotary polygonal mirror, which is rotated at a high speed, deflects the collimated laser beam to the photosensitive medium. The deflected beam is passed through an fθ lens, and the fθ lens focuses the deflected beam to form a light spot on the photosensitive medium surface. In the above laser scanning device, the collimator lens, the rotary polygonal mirror, and the fθ lens constitute the scanning optical unit.
In an image forming apparatus including the laser scanning device, the photosensitive medium surface is scanned in a main scanning direction by the laser beam from the laser scanning device. In a synchronous manner with the time the main scanning is performed, the photosensitive medium is rotated around its rotation axis, and the photosensitive medium surface is scanned in a sub-scanning direction by the laser beam from the laser scanning device. Hence, in the image forming apparatus, an electrostatic image is formed on the photosensitive medium surface by using the laser scanning device.
When the optical scanning device, like the above laser scanning device, is actually operated over an extended period of time, it is inevitable that a scanned-surface focal-point position of the light beam focused by the scanning optical unit deviates from a design-value position due to environmental changes, in particular, due to temperature changes of the scanning optical unit and its neighboring locations. In a certain case, the temperature changes cause the thermal deformation of the elements of the optical scanning device, and the light spot, which is formed on the scanned surface by the scanning optical unit, is larger than a required diameter. The contrast of the resulting image on the scanned surface will be lowered due to the focal-point deviation of the laser beam, and this will degrade the quality of the resulting image that is formed by the image forming apparatus using the optical scanning device.
As disclosed in Japanese Patent Publication No. 2692944, a scanning optical device that is provided with a temperature compensation unit for eliminating a scanned-surface focal-point deviation of the light beam due to a temperature change of the scanning optical device is known. In the temperature compensation unit of the above document, a focusing-condition detection means detects a focusing condition of the laser beam on the scanned surface and outputs a detection signal indicative of the focusing condition. A corrector lens is movably provided in the collimator lens of the scanning optical device. When a temperature detection means detects a temperature change of the scanning optical device, movement of the corrector lens along the optical axis relative to the scanned surface is controlled by a feedback loop based on the focusing-condition detection signal, until the focusing-condition detection means detects a desired focusing condition of the laser beam on the scanned surface in which the focal-point deviation of the light beam is cancelled. Alternatively, the temperature compensation unit of the above document may be achieved by either the movement of the collimator lens along the optical axis or the movement of the laser light source.
Further, as disclosed in Japanese Laid-Open Patent Application No. 4-107581, a scanning optical device that is provided with a temperature compensation unit is known. In the conventional temperature compensation unit of the above document, a focusing-condition detection means detects a focusing condition of the laser beam on the scanned surface of a photosensitive medium by outputting a detection signal indicating the focusing condition. A temperature control means, including the Peltier element, is provided on the laser light source. An automatic focusing device including a corrector lens movably provided therein is operated in response to a control signal output by the temperature control means, and the automatic focusing operation of the automatic focusing device is controlled in response to the detection signal output by the focusing-condition detection means, such that the corrector lens is moved along the optical axis by the automatic focusing device and the focusing-condition detection means detects a desired focusing condition of the laser beam on the scanned surface.
However, in the conventional scanning optical devices of the above documents, when a temperature change of the scanning optical device is detected, the automatic focusing operation must be performed to attain the desired focusing condition of the laser beam on the scanned surface. The configuration of the conventional scanning optical devices and the automatic focusing operation thereof are complicated, and much time is taken to reach the desired focusing condition of the laser beam by the automatic focusing operation. Further, the movement of the corrector lens is directed to elimination of only one of a main-scanning-direction focal-point deviation and a sub-scanning-direction focal-point deviation caused by the temperature change. If a focal-point position of the laser beam on the scanned surface that is optimum with respect to the main-scanning direction can be achieved, the resulting focal-point position of the laser beam on the scanned surface is not necessarily suitable with respect to the sub-scanning direction.